(174bm) Experimental Study of Cation Exchange Membrane Performance in Intensified Chlor-Alkali Electrolysis

Membrane chlor-alkali electrolysis is one of the largest industrial electrochemical processes producing chlorine, sodium hydroxide, and hydrogen. These products are the chemical building blocks in the manufacturing of many daily used products such as PVC, polyester textiles, pesticides, paints, detergents, paper, glass, personal care products, computers, smartphones, pharmaceuticals, etc.

The membrane, placed between the anode and cathode compartments, has three main functions. One function is to separate chlorine from hydrogen gas to prevent formation of an explosive mixture. The membrane also enables a zero (finite) gap configuration to remove the solution resistance to reduce the cell potential. The most important function of these cation-selective membranes, is the ability of the membrane to prevent co-ion (chloride and hydroxide ions) transport through the membrane. This is defined as membrane perm-selectivity. Undesired migration of hydroxide ions not only causes product contamination but also leads to a lower current efficiency of the process. Therefore, the industry uses bilayer membranes with a perfluorosulfonic acid layer at the anolyte side and a very thin carboxylic acid layer at the catholyte side. The sulfonic layer has a good conductivity, while the carboxylic acid layer especially prevents the migration of the hydroxide ions. However, blister formation can occur between the sulfonate and carboxylate layers due to an imbalance water transport across the boundary layer, and high local current density [1]. The blister formation limits electrical current density operation up to 8 kA/m² and this leads to a higher investment cost. Maximizing the use of the membrane will reduce the investment cost since cation selective membrane is the most expensive part of the electrochemical cell in the long run.

One main interest is to intensify the process by increasing the maximum current density, and the ambitious target would be 30 kA/m². The chlor-alkali industry is also energy intensive with electricity being over 50% of the operating cost. Both electrical current efficiency and cell potential determine the power consumption, which is directly related to both membrane perm-selectivity and membrane resistance. Monolayer sulfonate membrane has a lower membrane resistance, but it has a lower membrane selectivity than a bilayer membrane. High current density operation can increase membrane perm-selectivity [1, 2]. However, information on membrane perm-selectivity on the monolayer membrane at high current densities remains scarce.

In this study, membrane perm-selectivity is measured as a function of current density up to 25 kA/m² for both monolayer and bilayer cation selective membranes. We focus on the back migration of hydroxide ions using two types of experimental setups. One experimental setup is a laboratory scale chlor-alkali setup with a zero-gap cell, in whichthe anolyte pH is controlled through the addition of 1 M hydrochloric acid. At the anode, oxygen evolution occurs as a side reaction. Inline gas chromatography is installed to measure the produced oxygen. The other setup is a four electrode setup, which enables operation at very high current densities. In this setup the anolyte consists of sodium perchlorate to avoid chlorate formation. The permselectivity is measured by monitoring the change in pH of the anolyte.

This presentation consists of the results of the experiments and the comparison of the membrane perm-selectivity using the chlor-alkali setup and the comparable sodium perchlorate-sodium hydroxide system. Also, the effect of high current densities for both monolayer and bilayer membrane are discussed.

[1] T. F. O’Brien, T. V. Bommaraju, and F. Hine, Handbook of Chlor-Alkali Technology Volume I : Fundamentals. Springer, 2005.

[2] P. Granados Mendoza, S. Moshtarikhah, A. S. Langenhan, M. T. de Groot, J. T. F. Keurentjes, J. C. Schouten, and J. van der Schaaf, “Intensification of the chlor-alkali process by using a spinning disc membrane electrolyzer,” Chem. Eng. Res. Des., vol. 128, pp. 120–129, 2017.